Evolution

by Stephen Baxter

  In human times the sun had been embedded in the body of the huge flat disk, so that the Galaxy had been seen edge-on, its glory diminished by the obstructing dust clouds that littered the disk. But now the sun, following its slow orbit around the core, had sailed out of the Galaxy’s plane. Compared to the random scattering of a few thousand lamps that had marked man’s sky, this was like glimpsing the lights of a hidden city.

  Ultimate cowered.

  A bony hook rose in the sky. It was the Moon, of course, an old Moon, tonight a narrow crescent. The same patient face that had peered down on Earth since long before the birth of man was all but unchanged across half a billion years. And yet this thin crescent Moon shone more brightly over the new supercontinent than it had over the more equable lands in the past. For the Moon shone by reflected sunlight — and the sun had grown brighter.

  Had she known where to look Ultimate might have made out a dim smudge in the sky away from the Galaxy’s disk, easily visible on the clearest nights. That remote smudge was the great galaxy known as Andromeda, twice the size of its neighbor. It was still a million light-years distant from Earth’s Galaxy — but in human times it had been twice as far away as that, and even then it had been visible to the naked eye.

  Andromeda and the Galaxy were heading for a collision, still another half-billion years distant. The two great star systems would pass through each other like mingling clouds, with direct collisions between stars rare. But there would be a vast gush of star formation, an explosion of energy that would flood the disks of both galaxies with hard radiation. It would be a remarkable, lethal light show.

  But by then there would be little left alive on Earth itself to be troubled by the catastrophe. For the brightening of the sun was life’s final emergency.

  Morning came with its usual stark suddenness. Scuttling lizards and insects disappeared into the nooks and crannies where they would ride out the day, waiting for the richer opportunities of evening.

  The baby mewled. Her fur stuck up in clumps, and the pucker where the belly-root should sit looked inflamed. She kept up her complaints, her bulbous little head turning to and fro, until Ultimate had chewed some more liverwort and dribbled it into her mouth. Cactus, too, was grumbling, picking dirt and bits of dried shit from her fur.

  This morning it didn’t seem such a good idea to be out here in the middle of nowhere, so far from home. But as she held her baby Ultimate knew she had to stay away from the Tree — stay away, or lose her baby. She clung to that one irreducible fact.

  Ultimate and Cactus began to work their random way across the landscape, heading roughly away from the quarry. Just as they had yesterday, they ate where they could — though they found no water — and they avoided the rat-mouths and other hazards.

  And, at some point past noon, when the sun had begun its climb down the sky, Ultimate suddenly found herself facing the sphere once again.

  She had forgotten it existed. It did not occur to her to wonder how such an immense object might have gotten here from there, in the quarry.

  Cactus showed no interest, once she had figured out that you couldn’t eat the sphere. She passed on, grumbling to herself, picking bits of crimson dust out of her fur.

  Her baby asleep in her arms, Ultimate walked up to the sphere’s purple-black bulk. She sniffed it and, this time, tasted it. Again that unidentifiable electric tang subtly thrilled her. She lingered, somehow drawn. But the sphere offered her nothing.

  But suddenly Cactus was howling, thrashing on the ground. Ultimate whirled, crouching. Cactus’s left leg was somehow pinned, and blood spurted from her foot — and Ultimate heard the crunch of bone, as if poor Cactus’s limb had been taken in some vast mouth.

  But there was no mouth to be seen.

  No teeth and claws held Cactus. But slashes appeared on her chest and torso, dripping with startlingly bright blood, as if out of nowhere. Still she fought. She swung her fists, kicked, tried to bite even as she screamed. She was landing blows — Ultimate could hear the meaty sound of flesh being struck, and there were peculiar bits of discoloration in the air over Cactus, purple and blue. And the blood itself was starting to outline her assailant in crimson splashes. Ultimate could make out a long cylindrical torso, stubby legs, a wide, snapping mouth.

  But Cactus was losing her fight. Her legs and upper body became trapped under the shimmering mass. She turned to Ultimate, and reached out her hand.

  Instincts warred in Ultimate. It might have been different if she could have imagined how Cactus was feeling, the mortal fear that flooded through her. But Ultimate could not; empathy had been lost in mankind’s great shedding, along with so much else.

  She had hesitated too long.

  That great blurred mass raised itself up and came crashing down on Cactus. A thicker, richer blood gushed from the helpless posthuman’s mouth.

  Ultimate’s shock evaporated. With a squeal of terror she turned and ran, her squealing baby clutched to her chest, her feet and her free hand clattering over the dusty ground. She kept going until she came to an eroded ridge of crimson rock.

  She flung herself to the ground, and looked back. Cactus was still. Ultimate could make out nothing of the vast transparent thing that had destroyed her. But new creatures had emerged, as if from nowhere. They looked like frogs, with sprawling bodies, leathery amphibian skin, splayed, clawed feet, and wide mouths equipped with needle-sharp teeth for rending and gouging. Already the first of them had opened up Cactus’s chest and was feasting on the still-warm organs within.

  The invisible predator had done its job. It lay exhausted in a pool of Cactus’s blood. It was too weary even to feed itself, and it relied on scraps brought to it by its greedy siblings. The meat could be seen being shredded by its grinding teeth and then passing into its gullet and stomach, where digestive processes would begin to absorb and transform it.

  As the world had emptied and been eroded flat, the lack of cover was the killer. In a landscape like a pool table, you just couldn’t hide a one-ton salamander, even if it was painted as red as the rocks. That was why most of the big animals had quickly disappeared, outcompeted by their smaller cousins.

  But these creatures had adopted a novel strategy: the ultimate camouflage. The great redesign had taken many tens of millions of years.

  Invisibility — or at least transparency — had been a strategy adopted by some fish in earlier times. There were transparent substitutes for most of the body’s biochemicals. A substitute had to be found for hemoglobin, for example, the bright red protein in blood cells that combined with oxygen to transport that vital substance through the body.

  Of course no land-going creature could ever be truly invisible. Even in these arid times all animals were essentially bags of water. If you were actually immersed in water — where those long-extinct fish had once swum — something approaching true invisibility could be achieved. But light moved differently through air and water; in the air the final land-going “invisible” actually looked like a big bag of water sitting in the dirt.

  Still, it worked pretty well. As long as you kept still you were hard to see — just a mistiness, a slight distortion here and there that might easily be mistaken for a bit of heat shimmer. You could huddle against a rocky outcrop, ensuring that you presented only your least visible angles to any prey. You even had fur, transparent-like fiber-optic cable, which transmitted bits of background color to baffle your prey further.

  But even so, few species had adopted the stratagem, for invisibility was a blight.

  Every invisible was blind, of course. No transparent retina could trap light. On top of that the creature’s biochemistry, limited by the use of transparent substances, was a lot less efficient. And there was no shielding, even for its innermost parts, from the ferocious light, heat, and ultraviolet radiation from the sun, or from the cosmic radiation that had always battered the planet despite its great shield of magnetism. Its organs were transparent, but not transparent enough to let through all the damagin
g radiation.

  Already Cactus’s killer was in agony, and soon the cancers developing in its transparent gut would kill it. And it was neotenous. It would die without reaching puberty. None of the invisible kind had ever lived long enough to breed true, nor would their genetic material, damaged by radiation, ever have been able to produce a viable offspring.

  Sickly, helpless from birth, these wretched creatures began dying before they emerged from their eggs.

  But that didn’t matter, not from the point of view of the genes, for the family benefited.

  This amphibian species had reached a compromise. Most of its young were born as they always had been. But perhaps one in ten was born invisible. Like the sterile workers in a hive, the invisible lived through its brief, painful life and died young, all for a single purpose: to retrieve food for its siblings. Through them — through their offspring, not its own — the invisible’s genetic legacy would live on.

  It was an expensive strategy. But it was better to sacrifice one in ten of each generation to a brief life of agony than to succumb to extinction.

  The presence of food in its stomach and waste in its lower gut made the invisible easy to spot, of course. So when they were hungry again its siblings would starve it, waiting for all the waste to pass out of its system, rendering it as transparent as possible. And then they would set it to work once more, under the lethal sun, hoping to have it snatch one more meal for them before it died.

  The sphere had made its own observations of these events.

  The sphere was a living thing, and yet it was not. It was an artifact — and yet it was not that either. The sphere had no name for itself, or for its kind. Yet it was conscious.

  It was one of a great horde that now spanned the stars, in a great belt of colonization that swept around the Galaxy’s limb. And yet the sphere had come here, to this ruined world, seeking answers.

  Memories stretched deep. Among the sphere’s kind, identity was a fluid thing, to be split and shared and passed on through components and blueprints. The sphere could think back, deep through thousands of generations, but it was a memory trail that ended in mist. The replicating hordes had forgotten where they came from.

  In its way, the sphere longed to know. How had this great star-spanning swarm of robots first originated? Had there been some form of spontaneous mechanical emergence, cogs and circuits coming together on some metallic asteroid? Or had there been a Designer, some other, who had brought the progenitors of these swarming masses into being?

  For a million years the sphere had studied the distribution of the replicators through the Galaxy. It wasn’t easy, for the great disk had rotated twice since the origin of its kind, and the stars had swum about, smearing the robot colonists across the sky. Great mathematical models had been built to reverse that great turning, to restore the stars as they once must have been, to map back the replicators’ half-forgotten expansion.

  And at last the sphere had converged on this system, this world — amid a handful of others — as the putative origin. It had found a world of organic chemistry and creatures interesting in their way. But it was a dying world, overheated by its sun, the life-forms restricted to the fringes of a desert continent. There was no sign of organized intelligence.

  And yet, here and there, the ancient rocks of the supercontinent had been marked deliberately, it seemed to the sphere, with cuts and gouges and great pits. Once there had been mind here, perhaps. But if so it was vanished from these wretched, crawling creatures.

  The sphere represented a new order of life. And yet it was like a child, wistfully seeking its lost father. The last traces of the Martian robots’ original blueprint, assembled by long-dead NASA engineers in computer laboratories in California and New England and much modified since, had been lost. It was somehow appropriate that this greatest, and strangest, of all of mankind’s legacies should have been created entirely accidentally — and that those created should have been abandoned to their fate.

  There was nothing more to be learned here. With an equivalent of a sigh, the sphere leapt to the stars. The small world dwindled behind it.

  Ultimate huddled in the dirt until the scavenging siblings had finished feeding. Then she stumbled away, clutching her baby, not even noticing that the sphere had vanished.

  III

  Ultimate kept heading west, away from the borametz quarry.

  At night she wedged herself with her infant into crevices between rocks, trying to emulate the comforting enclosure of the Tree’s cocoon. She ate whatever she could find — half-desiccated toads and frogs buried in the mud, lizards, scorpions, the flesh and roots of cacti. She fed the child a chewed-up pulp of meat and vegetable matter. But the child spat out the coarse stuff. She was still missing her belly-root, and she mewled and complained.

  Ultimate walked, and walked and walked.

  She had no strategy in mind save to keep moving, to keep her infant out of the chemical clutches of the Tree, and to wait to see what turned up. If her thinking had been sophisticated enough she might have hoped to find more people, somewhere she could stay, maybe even a community that lived independently of the Trees.

  It would have been a futile hope, for there were no more such communities anywhere on Earth. She didn’t know it, but she had nowhere to go.

  The land began to rise slowly. Ultimate found herself walking on coarse sand and gravel fans.

  After half a day of this she came to a place of low, smooth-shouldered hills. She could see how these eroded stumps went marching off to the horizon, to north and south, for kilometer after kilometer, all the way to the dust-laden horizon and beyond. She was walking through the remnant of a once-great mountain chain, thrown up by the ancient stitching together of the continents. But the dust-laden winds of New Pangaea had long since worn down the mountains to these meaningless stumps.

  When she looked back she could see her own footprints, accompanied by the scraping of knuckles, marked by the messy places where she had stopped to feed or make waste or sleep. They were the only trail through these silent hills.

  It took her two days to cross the mountains.

  After that the land began to descend again.

  On the plain, a little more vegetation grew. There were spiky trees with gnarled branches and clumps of needlelike leaves, like bristlecone pines. Around their roots sheltered a few leaping mice — hardy rodent survivors, ferocious conservers of water — and many, many lizards and insects. She chased tiny things like geckos and iguanas, and munched their flesh. But on this looser ground Ultimate had to be cautious, watching for rat-mouths embedded in the ground, and for the quivering, invisible mass of an ambush hunter.

  As the land descended further, the view to the west opened up. She saw a great plain. Beyond a kind of coastal fringe, the land was white, white as bone, a sheet that ran all the way to a knife-sharp, geometrically flat horizon. A thin wind moaned in her face. On its breath, she could already taste the salt. Nothing moved, as far as she could see.

  She had come to a fragment of the dying inland ocean. There was still water out there — it took a long, long time to dry out a sea — but it was a narrowing strip of water so saline it was all but lifeless, and it was framed by this great white rim of exposed salt flats, a sheet that extended to her horizon.

  Tucking her baby’s face into the fur on her breast, Ultimate continued her dogged descent.

  She reached the place where the salt began. Great parallel bands showed where water had once lapped. She scooped up a bit of the salty dirt and licked it. She spat out the bitter stuff immediately. There was vegetation here, tolerant of the salty soil. There were small, spiky yellow shrubs that looked like the desert holly and honeysweet and spurge that had once clung to life in the Californian deserts of North America. Experimentally she broke off a bit of the holly’s foliage and tried to chew it, but it was too dry. Frustrated, she hurled the bit of twig away over the salt.

  And then she saw the footprints.

  Curious,
she fit her own feet into the shallow indentations in the ground. Here had been toes, here a scuffing that might have been caused by a resting knuckle. The prints could not have been made recently. The mud was baked hard as rock, and her own weight left no mark.

  The prints set out, straight as an arrow, across the salt pan, marching on toward the empty horizon. She followed for a step or two. But the salt was hard and harsh and very hot, and when it got into minor cuts and scrapes on her feet and hands it stung badly.

  The footprints did not turn back. Whoever had made them had not returned. Perhaps the walker had been intent on reaching the ocean proper, on walking all the way across North America: After all, there were no barriers now.

  She knew she could not follow, not into the belly of this dead sea.

  And it would have made no difference even if she had. This was New Pangaea. Wherever she went she would have found the same crimson ground, the same searing heat.

  She stayed on the desolate, silent beach for the rest of the day. As it descended the overheating sun grew huge, its circular shape quivering. Its harsh light turned the salt plain a washed-out pink.

  This had been the last significant journey ever undertaken by any of her ancient, wandering lineage. But the journey was over. This parched, dead beach had been the farthest point of all. The children of humanity had done with exploring.

  As the light failed she turned away and began to walk up the sloping ground. She did not look back.

  In the years after Ultimate’s death Earth would spin on, ever more slowly, its waltz with its receding Moon gradually running down.

  And the sun blazed ever brighter, following its own hydrogen logic.

  The sun was a fusion furnace. But the sun’s core was becoming clogged with helium ash, and the surrounding layers were falling inwards: The sun was shrinking. Because of this collapse the sun was getting hotter. Not by much — only by around 1 percent every hundred million years — but it was relentless.